I used quen to lessen his damage then set a yrden up, not the circle but the zappy one. When he comes close and gets hit with that it stuns him so he cant move and then I used a stream of igni on him when he was stunned and repeated that process.
Tip: Superior Petri's Philter + Level 3 Aard Sweep + Piercing Cold mutation (from Blood and Wine). You can instakill so many enemies with that.